Tetrafluoroethylene thermoprocessable copolymers

ABSTRACT

Tetrafluoroethylene (TFE) thermoprocessable copolymers, consisting of: 
     (A) from 0.1 to 15% by moles of a fluorodioxol of formula ##STR1## wherein R F  is a perfluoroalkyl having from 1 to 5 carbon atoms; X 1  and X 2 , equal to or different from each other, are --F or --CF 3  ; Z is selected from --F, --H and --Cl; 
     (B) from 0 to 15% by moles of a perfluorinated monomer selected from hexafluoropropene (HFP) and a perfluoralkylvinylether of formula CF 2  ═CF--OR&#39; f  wherein R&#39; f  is a perfluoroalkyl C 2  -C 4  or their mixtures; excluding the value 0 for the amount of the monomer (B); and 
     (C) TFE forming the remaining part to 100%; with the proviso that the total amount of the monomers (A) and (B) is lower than or equal to 20% by mole.

This is a continuation of U.S. application Ser. No. 08/582,916, filedJan. 4, 1996 ABN.

The present invention relates to tetrafluoroethylene thermoprocessablecopolymers. More particularly, the present invention relates totetrafluoroethylene thermoprocessable copolymers with a fluorodioxolehaving improved mechanical properties, in particular at hightemperatures, combined with superior optical properties.

It is known that polytetrafluoroethylene (PTFE) shows a very highmelting viscosity, whereby it cannot be worked according to thetechniques usually employed for thermoprocessable polymers (extrusion,molding, injection, etc.).

To obviate this drawback it is known to copolymerize tetrafluoroethylene(TFE) with hexafluoropropene (HFP) (see for instance U.S. Pat. No.2,946,763). In order to succeed in combining acceptable mechanicalproperties with a melting low viscosity and therefore with goodprocessability, it is necessary to introduce high HFP amounts, generallyaround 7-11% by moles. However, so high amounts of HFP cause asignificant fall in the second melting temperature and consequently aclear worsening of properties at high temperature, in particular tensileand creep properties. Therefore the continuous working temperature fallsfrom values of about 260° C. for PTFE to about 200° C. for TFE/HFPcopolymers.

Other TFE thermoprocessable copolymers are those wherein TFE iscopolymerized with a perfluoroalkylvinylether (PAVE), and in particularwith perfluoropropylvinylether (PPVE) (see for instance U.S. Pat. No.3,635,926). Such copolymers allow to obtain a satisfactory balancebetween mechanical properties and processability with PPVE amountsaround 2-3% by moles.

Finally to try to obviate the drawbacks described above for the TFE/HFPcopolymers, terpolymers have been suggested wherein TFE is copolymerizedwith 4-12% by weight of HFP and 0.5-3% by weight of PPVE (see U.S. Pat.No. 4,029,868).

Even though the employment of PPVE has substantially raised the TFEthermoprocessable copolymers properties, in particular with reference tothe maximum working temperature and to the processability, thus greatlyenlarging the application field, it is however felt the need from onehand to further broaden the properties range and from the other hand tomake easier the conditions of the copolymers synthesis. It is indeedknown that PPVE shows poor reactivity and needs complex processes ofrecovering of the unconverted monomer (see for instance GB patent1,514,700).

Fluorinated dioxols of various types are known, having general formula:##STR2## wherein Y₁, Y₂ are --H, --F or --Cl: Y₃, Y₄ are --F or --CF₃(see for instance U.S. Pat. No. 3,865,845, EP 76,581, EP 80,187, EP95,077, EP 73,087). Such compounds can be used for preparinghomopolymers of copolymers with other fluorinated monomers. Inparticular both amorphous and crystalline copolymers are described andprepared between the above mentioned dioxols and TFE (see for isntanceEP Patents 73,087, EP 95,077, U.S. Pat. No. 4,558,141).

In European patent application No. 94109782.6 in the name of theApplicant new fluorodioxols of formula: ##STR3## are described, whereinR_(F) is a perfluoroalkyl having from 1 to 5 carbon atoms; X₁ and X₂,equal to or different from each other, are --F or --CF₃. Such dioxolscan be used for preparing homopolymers and copolymers with otherfluorinated monomers. In particular TFE thermoprocessable terpolymersare described wherein TFE is copolymerized with a fluorodioxol offormula (I) and with perfluoromethylvinylether. In examples 9 and 10crystalline copolymers are moreover prepared from TFE and2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxol, the latter being presentin amounts equal to 1.1% and 3.3% by moles respectively. Thecopolymerization process was carried out in solution in oganic solvent(CCl₂ FCF₂ Cl). The copolymers thus obtained have shown a very highmelting viscosity, with a Melt Flow Index value unmeasurable andtherefore not thermoprocessable. No characterization was supplied asregards the optical properties of such copolymers.

The Applicant has now surprisingly found that the copolymerization of afluorodioxol as hereinunder defined by TFE, optionally in associationwith HFP and/or a perfluoroalkylvinylether, for instanceperfluoropropylvinylether and perfluoromethylvinylether, allows toobtain TFE thermoprocessable polymers with improved mechanicalproperties, in particular at high temperatures. Such properties arecombined with superior optical properties, as shown by radiationtransmission measurements (transmittance and haze). This combination ofproperties results unexpectedly higher than the one obtainable withknown modifying comonomers, in particular with respect to the knownfluorodioxols.

The fluorodioxols of the present invention determine, with the sameamount introduced, a more marked fall of the second melting temperaturesurprisingly combined with a maintenance of the tensile properties andof resistance to brittle fracture at high temperatures.

Conversely, experiments carried out by the Applicant have unexpectedlyshown that the fluorodioxols of the present invention have a highermodifying power than the known fluorodioxols of the prior art, and thisallows to prepare copolymers having a lower content of modifyingcomonomer, which, the second melting temperature being equal, showsuperior engineering properties, in particular yield stress and improvedheat creep values.

The reactivity of the fluorodioxols of the present invention is so highthat there is no need of complex processes for recovering the monomer.

Subject matter of the present invention are thereforetetrafluoroethylene (TFE) thermoprocessable copolymers comprising:

(A) from 0.1 to 15%, preferably from 0.5 to 9%, by moles of afluorodioxol of formula ##STR4## wherein R_(F) is a perfluoroalkylhaving from 1 to 5 carbon atoms; X₁ and X₂, equal to or different fromeach other, are --F or --CF₃ ; Z is selected from --F, --H, --Cl;

(B) from 0 to 15%, preferably from 0 to 10% by moles of a perfluorinatedmonomer selected from hexafluoropropene (HFP) andperfluoroalkylvinylethers of formula CF₂ ═CF--OR'_(f), wherein R'_(f) isa perfluoroalkyl C₂ -C₄, or their blends;

(C) TFE, forming the remaining part to 100%;

with the proviso that the total amount of the monomers (a) and (b) islower than or equal to 20% by moles, preferably lower than or equal to12% by moles.

Preferably, in formula (I) X₁, X₂ and Z are --F, R_(f) is preferably--CF₃, --C₂ F₅, or --C₃ F₇. Fluorodioxols of formula (I) wherein R_(f)is --CF₃ or --C₂ F₅ and X₁, X₂ and Z are --F, are particularlypreferred.

Among the perfluoroalkylvinylethers of formula CF₂ ═CF--OR'_(f),perfluoropropylvinylether (PPVE) is particularly preferred.

The fluorodioxols of formula (I) are described in European patentapplication No. 94109782.6 in the name of the Applicant, the content ofwhich is herein incorporated by reference. In case Z is --F, they can beprepared by the following process, comprising:

(a) to react at a temperature comprised between -140° and +60° C.(preferably between 110° and -20° C.) a dioxol of formula: ##STR5##wherein X₁ and X₂ have the meaning indicated above, with afluoroxy-compound of formula R_(f) OF, wherein R_(f) has the meaningindicated above, thus obtaining a dioxolane of formula: ##STR6## (b) todehalogenate the dioxolane (III) according to known techniques, byreacting it with a metal in an aprotic dipolar solvent.

The dioxols of formula (II) are known compounds; they can be preparedfor instance according to EP Patent application 460,946. Alsofluoroxycompounds R_(f) --OF are known products. CF₃ OF can be prepared,for instance, according to what described by G. H. Cady and K. B.Kellogg in J. Am. Chem. Soc. 70, 3986, 1948, the higher homologues bythe process described in U.S. Pat. No. 4,827,024.

An alternative process to the preceding one for preparing fluorodioxolsof formula (I) wherein Z is --F, described in European patentapplication No. 94109782.6 as well, comprises:

(a) to react, at a temperature comprised between -140° and +60° C.(preferably between -110° and -20° C.), an olefin of formula:

    R.sub.f O--CCl═CFCl                                    (IV)

wherein R_(f) is defined as above, with a bis-fluoroxy-compound havingthe formula:

    CX.sub.1 X.sub.2 (OF).sub.2                                (V),

thus obtaining the dioxolane of formula (III);

(b) to dehalogenate the dioxolane (III) as described above.

The olefin (IV) can be obtained by reacting CCl₂ ═CCl₂ with R_(f) OF soas to obtain the compound R_(f) --OCCl₂ --CFCl₂, which, bydechlorination reaction with powdered zinc in organic solvent, suppliesthe olefin (IV).

A further process for preparing fluorodioxols of formula (I) when Z is--F, described in European patent application No. 94109782.6 as well,comprises reacting at a temperature comprised between 50° and 150° C. adioxolane of formula: ##STR7## optionally in admixture with a dioxolaneof formula: ##STR8## wherein R_(f) is defined as above, with KOH in thesolid state, with consequent dehydrochlorination and formation offluorodioxol.

The dioxolane (VI) can be obtained by reacting R_(f) OF withtrichloroethylene so as to obtain the R_(f) O--CHCl--CFCl₂ compound,which is then dechlorinated with powdered zinc in organic solution. Theolefin of formula R_(f) --OCH═CFCl is thus obtained, which is finallyreacted with CX₁ X₂ (OF)₂, as described above for the olefin (IV),obtaining the dioxolane (VI).

The dioxolane (VII), in admixture with the dioxolane (VI), can beprepared as follows. The olefin CHCl═CHCl is reacted with CX₁ X₂ (OF)₂,obtaining the dioxolane having the formula: ##STR9## which, bydehydrohalogenation with solid KOH, gives the dioxol of formula:##STR10## This is at last reacted with R_(f) OF, thus obtaining amixture of the dioxolanes (VI) and (VII).

The synthesis of fluorodioxols (I) can utilize, similarly to theprocesses already shown, the dichloroethylene or trichloroethylene,which are reacted with hypofluorites of formula:

    R.sub.f --OF and CX.sub.1 X.sub.2 (OF).sub.2,

so as to obtain, by alternating reactions of dechlorination anddehydrochlorination, the various reaction intermediates described above.

Similarly to what described above for Z═--F, when Z is --Cl or --H, thefluorodioxols of formula (I) can be prepared according to the followingmethod. 1,1-dichloroethylene CH₂ ═CCl₂ is reacted with hypochloriteR_(f) OCl and the reaction product is dehydrochlorinated as describedabove, so that the olefin of formula R_(f) --OCCl═CHCl is obtained. Thislast is reacted with CX₁ X₂ (OF)₂, as described above for the olefin(IV), obtaining the dioxolane having the formula: ##STR11##

The dioxolane (X) can be dehydrochlorinated with solid KOH by supplyingthe fluorodioxol (I) with Z═--Cl, or it can be dechlorinated withpowdered Zn in organic solvent to give the fluorodioxol (I) with X═--H.

The copolymers of the present invention show a melting viscosity such asto make them thermoprocessable according to conventional techniques,with measurable Melt Flow Index (MFI) values. In particular MFI,measured according to standard ASTM D 1238, is generally comprisedbetween 0.5 and 50 g/10', preferably between 1 and 30 g/10'.

The copolymers of the present invention can be prepared according toknown techniques, by copolymerization of the corresponding monomers, insuspension in organic medium or in aqueous emulsion, in the presence ofa suitable radicalic initiator, at a temperature comprised between 0°and 150° C., preferably between 20° and 100° C. The reaction pressure isgenerally comprised between 0.5 and 100 bar, preferably between 5 and 40bar.

Among the various radicalic initiators can be used in particular:inorganic peroxides soluble in water, such as for instance persulphatesand ammonium or alkaline metals perphosphates, in particular ammonium orpotassium persulphate; organic or inorganic redox systems, such asammonium persulphate/sodium sulphite, hydrogenperoxide/aminoiminomethansulphinic acid; bis-acylperoxides of formula(R_(f) --CO--O)₂, wherein R_(f) is a (per)haloalkyl C₁ -C₁₀, or aperfluoropolyoxyalkylenic group, such as for instancebis(perfluoropropionyl)peroxide; dialkylperoxides of formula (R_(f)--O)₂, wherein R_(f) is a perhaloalkyl C₁ -C₁₀, such as for instancediterbutylperoxide (DTBP); etc.

In case of copolymerization in suspension, the reaction medium is formedby an organic phase, to which water is usually added to favour thedispersion of the heat developing during the reaction. As organic phase,halogenated hydrocarbons, in particular hydrogen(chloro)fluorocarbonsand chlorofluorocarbons; fluoropolyopxyalkylenes; etc., can be employed.

In case of the (co)polymerization in aqueous emulsion, the presence of asuitable surfactant is required. The most commonly used are fluorinatedsurfactants of formula:

    R.sub.f --X.sup.- --M.sup.+

wherein R_(f) is a (per)fluoroalkylic chain C₅ -C₁₆ or a(per)fluoropolyoxyalkylenic chain, X⁻ is --COO⁻ or --SO₃ ⁻, M⁺ isselected from: H⁺, NH₄ ⁺, an alkaline metal ion. Among them, ammoniumand/or sodium perfluoro-octanoate; (per)fluoropolyoxyyalkyleneterminated with one or more carboxylic groups, etc. can be mentioned.

The process object of the present invention can be advantageouslycarried out in the presence of perfluoropolyoxyalkylenes emulsions ormicroemulsions, according to U.S. Pat. Nos. 4,789,717 and 4,864,006, oralso of fluoropolyoxyalkylenes microemulsions having hydrogenated endgroups and/or hydrogenated repeating units, according to EP patentapplication 625,526.

In order to check the molecular weight of the final product, andtherefore the melting viscosity, suitable chain transfer agents areadded to the reaction system, such as: hydrogen, hydrocarbons;optionally containing halogens, for instance methane, ethane,chloroform, methylenechloride, etc.; esters, ethers or aliphaticalcohols, for instance methanol, ethanol, diethylmalonate, etc. Thetransfer agent is sent into the reactor at the beginning of thereaction, or continuously on in discrete amounts in the course of thepolymerization. The amount of the chain transfer agent utilized canrange within rather wide limits, depending on the molecular weightdesired, of the effectiveness of the transfer agent itself and of thereaction temperature.

Some examples of the present invention are reported hereinafter, whosepurpose is merely illustrative and not limitative of the inventionitself.

EXAMPLE 1

A 5 l AISI 316 steel chromated autoclave, equipped with a stirrerworking at 650 rpm was evacauted and 3.0 l of demineralized water, and2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxol (TTD), having the formula##STR12## are introduced therein in amounts equal to 0.67 g/l H₂ O. Amicroemulsion of perfluoropolyoxyalkylenes was then added, obtainedaccording to Example 1 of U.S. Pat. No. 4,864,006, in such amounts as toobtain a concentration of perfluoropolyoxyalkylenic surfactant equal to2.0 g/l H₂ O. The autoclave was brought to the working temperature of75° C. and then 0.24 absolute bar of ethane were loaded, acting as chaintransfer agent. The autoclave was then brought to the working pressionof 21 absolute bar by feeding a TFE/TTD gaseous mixture in molar ratio54,55/1. Before beginning the reaction, the gaseous phase, analyzed bygaschromatograph, showed the following composition (% by moles): 97.9%of TFE, 2.1% of TTD. By means of a metering pump the initiatopr was thenfed, consisting in a solution of potassium persulphate (KPS) havingconcentration equal to 0.00315 moles/l, with a flow rate of 88 ml/hour.During the reaction, the working pressure was kept constant bycontinuously feeding the reaction mixture TFE/TTD having the compositionindicated above. The reaction was stopped after 1560 g of TFE/TTDmixture were fed. The gaseous phase present in the autoclave had at theend of the reaction the following composition (% by moles): 98.2% ofTFE, 1.8% of TTD. After the reactor was cooled at room temperature, theemulsion was discharged and coagulated by addition of a 65% by weightHNO₃ aqueous solution. The resulting polymer was separated, washed withdemineralized water and dried. The polymer was characterized as reportedin Table 1.

The composition of the polymer was determined by mass balance. Thesecond melting temperature (T_(2m)) was determined by scanningdifferential calorimetry (DSC), the Melt Flow Index according tostandard ASTM D-1238-52T, with a 5 kg load.

The optical properties, that is, haze and transmittance were measuredaccording to standard ASTM D 1003.

The mechanical properties were measured at 23° and 250° C., according tostandard ASTM D 1708, with stretching rate of 50 mm/min, on sampleshaving a thickness of 1.58±0.08 nn, compression molded according tostandard ASTM D 3307-81. The reported values are the average of 3measurements.

EXAMPLE 2

Example 1 was repeated under the same conditions, except for the amountof ethane fed before beginning the reaction, which was equal to 0.52absolute bar. The characteristics of the obtained polymer are reportedin Table 1. Creep measurements were moreover carried out thereon,according to ASTM D 2990, on samples molded by compression andpretreated at 200° C. for 48 hours, by tensile stress at 275° C. with astrength of 1.6 MPa. The results are as well reported in Table 1.

EXAMPLE 3

Example 1 was repeated under the same conditions, except for the amountof ethane fed before beginning the reaction, which was equal to 0.57absolute bar. The characteristics of the obtained polymer are reportedin Table 1.

EXAMPLE 4 (Comparative)

In the same autoclave of Example 1, 3.0 l of demineralized water and amicroemulsion of perfluoropolyoxyalkylenes obtained according to Example1 of U.S. Pat. No. 4,864,006, in such amount as to obtain aconcentration of perfluoropolyoxyalkylenic surfactant equal to 2.0 g/lH₂ O, were introduced, after evacuation. The autoclave was brought tothe working temperature of 75° C. and then 0.57 absolute bar of ethanewere loaded, acting as chain transfer agent. The autoclave was thenbrought to the working pression of 21 absolute bar by feeding a gaseousmixture between TFE and 2,2,4,5-tetra-fluoro-1,3-dioxol (PD), having theformula ##STR13## with molar ratio TFE/PD equal to 54.55/l. Beforebeginning the reaction, the gaseous phase, analyzed by gaschromatograph,showed the following composition (% by moles): 98.2% of TFE, 1.8% of PD.By means of a metering pump the initiator was then fed, formed by asolution of potassium persulphate (KPS) having concentraion equal to0.00315 moles/l, with a 88 ml/hour, flow. During the reaction, theworking pressure was kept constant by continuously feeding the reactionmixture TFE/PD having the composition indicated above. The reaction wasstopped after 780 g of TFE/PD mixture were fed. The gaseous phase at theend of the reaction had the following composition (% by moles): 99.1% ofTFE, 0.9% of PD. The obtained polymer was characterized as reported inTable 1.

EXAMPLE 5 (Comparative)

In the same autoclave of Example 1, , 3.0 l of demineralized water andperfluoropropylvinylether (PPVE) in an amount equal to 3.67 g/l H₂ Owere introduced, after evacuation. A microemulsion ofperfluoropolyoxyalkylenes obtained according to Example 1 of U.S. Pat.No. 4,868,006, was then added, in such an amount to obtain aconcentration of perfluoropolyoxyalkylenic surfactant equal to 2.0 g/lH₂ O. The autoclave was brought to the working temperature of 75° C. andthen loaded with 0.35 absolute bar of ethane, acting as chain transferagent. The autoclave was then brought to the working pression of 21absolute bar by feeding a gaseous mixture TFE/PPVE with molar ratioequal to 54,55/l. Before beginning the reaction, the gaseous phase,analyzed by gaschromatograph, showed the following composition (% bymoles): 94.52% of TFE, 5.2% of PPVE. By means of a metering pump theinitiatopr was then fed, consisting in a solution of potassiumpersulphate (KPS) having concentration equal to 0.00315 moles/l, with aflow of 88 ml/hour. During the reaction, the working pressure was keptconstant by continuously feeding the reaction mixture TFE/PPVE havingthe composition indicated above. The reaction was stopped after 1560 gof TFE/PPVE mixture were fed. The gaseous phase at the end of thereaction had the following composition (% by moles): 94.3% of TFE, 5.7%of PPVE. After the reactor was cooled at room temperature, the emulsionwas discharged and coagulated by addition of a 65% by weight aqueoussolution of HNO₃. The polymer was characterized as reported in Table 1.

Hot creep measurements were carried out on said product as reportedabove. The results are as well reported in Table 1.

                  TABLE 1                                                         ______________________________________                                        EXAMPLE          1      2      3    4.sup.(*)                                                                          5.sup.(*)                            ______________________________________                                        Polymer composition                                                                            TFE    TFE    TFE  TFE  TFE                                  (% by mole)      98.2   98.2   98.2 98.2 98.2                                                  TTD    TTD    TTD  PD   PPVE                                                  1.8    1.8    1.8  1.8  1.8                                  MFI (g/10')      4      13     19   4    13                                     T.sub.2m  (° C.) 304.5 305.4 306.0 306.5 308.3                         Haze 41 39 45 50 37                                                           Transmittance 87 90 90 84 88                                                  Mechanical properties at 23° C.                                        Elastic modulus (MPa) 517 525 563 494 563                                     Yield point (MPa) 14.8 15.2 15.4 13.7 14.7                                    Stress at break (MPa) 32.3 26.2 23.3 22.5 28.6                                Elongation at break (%) 336 323 306 302 378                                   Mechanical properties at 250° C.                                       Elastic modulus (MPa) 28 26 32 29 36                                          Yield point (MPa) 2.15 2.2 2.3 2.1 2.3                                        Stress at break (MPa) 6.4 5.9 5.1 3.9 7.5                                     Elongation at break (%) 334 369 308 255 539                                   Mechanical properties at 275° C.                                       Elastic modulus (MPa)  24 26  26                                              Yield point (MPa)  1.75 1.8 -- 1.75                                           Stress at break (MPa)  4.2 3.9  5.2                                           Elongation at break (%)  401 379  535                                         Creep at 275° C. and 1.3 MPa                                           Elongation at 15 min (%)  5.4   5.8                                           Elongation at 60 min (%) -- 9.0 -- -- 10.0                                    Elongation at 48 hours (%)  21.0   25.5                                     ______________________________________                                         .sup.(*) comparative                                                     

EXAMPLE 6

Example 1 was repeated under the same conditions, except for the initialamount of TTD, equal to 0.6 g/l H₂ O, and for the molar ratio of the fedTFE/TTD mixture, equal to 61.5/l. The characteristics of the obtainedpolymer are reported in Table 2.

EXAMPLE 7

Example 4 was repeated under the same conditions, except for the amountof ethane fed before beginning the reaction, which was equal to 0.15absolute bar. The characteristics of the obtained polymer are reportedin Table 2.

                  TABLE 2                                                         ______________________________________                                        EXAMPLE             6        7.sup.(*)                                        ______________________________________                                        Polymer composition TFE 98.4 TFE 98.2                                           (% by mole) TTD 1.6 PPVE 1.8                                                MFI (g/10')         4        4                                                  T.sub.2m  (° C.) 308 308                                               Mechanical properties at 23° C.                                        Elastic modulus (MPa) 530 530                                                 Yield point (MPa) 15.2 14.0                                                   Stress at break (MPa) 31.9 31.8                                               Elongation at break (%) 342 360                                               Mechanical properties at 250° C.                                       Elastic modulus (MPa) 32 33                                                   Yield point (MPa) 2.4 2.0                                                     Stress at break (MPa) 8.8 9.8                                                 Elongation at break (%) 342 535                                             ______________________________________                                         .sup.(*) comparative                                                     

EXAMPLE 8

Example 1 was repeated but by using an amount of TTD of 0.427 g/l (H₂O).

The molar ratio in the feeding gaseous mixture between TFE/TTD was60.73/l.

The amount of ethane introduced was 0.375 bar.

The characteristics of the polymer so obtained are reported in Table 3and can be compared with the data of Example 4.

EXAMPLE 9 (Comparative)

Example 4 was repeated but with the following differences:

the molar ratio in the gaseous feeding mixtures TFE/PD was 45.95/l;

the ethane amount was 0.627 bar.

The data on the characterization of the polymer are reported in Table 3and can be compared with Example 2.

EXAMPLE 10 (Comparative)

Example 4 was repeated but with the following differences:

the molar ratio in the gaseous feeding mixtures TFE/PD was 45.95/l;

the ethane amount was 0.64 bar.

The data on the characterization of the polymer are reported in Table 3and can be compared with Example 3.

EXAMPLE 11

Example 1 was repeated but with the following differences: the ethaneamount was 0.336 bar.

The data on the characterization of the polymer are reported in Table 3.

EXAMPLE 12 (Comparative)

Example 11 was repeated but by using PD and a molar ratio TFE/PD of45.91/l; and the ethane amount was 0.619 bar.

The data on the characterization of the polymer are reported in Table 3and can be compared with Example 11.

                                      TABLE 3                                     __________________________________________________________________________    EXAMPLE       8    9.sup.(*)                                                                           10.sup.(*)                                                                           11   12.sup.(*)                               __________________________________________________________________________    Polymer composition                                                                         TFE 98.38                                                                          TFE 97.87                                                                           TFE 97.87                                                                            TFE 98.2                                                                           TFE 97.87                                  (% by mole) TTD 1.62 PD 2.13 PD 2.13 TTD 1.8 PD 2.13                        MFI (g/10')   4.3  11    17     8.5  8                                          T.sub.2m  (° C.) 306.1 305.6 306.0 305.3 305.7                         Haze 36 57 -- 41 51                                                           Transmittance 84 82 -- 83 85                                                  Mechanical properties at 23° C.                                        Elastic modulus (MPa) 570 560  570 590                                        Yield point (MPa) 15.8 14.6 -- 16 14.6                                        Stress at break (MPa) 28.9 15.5  29 18.1                                      Elongation at break (%) 330 170  344 250                                      Mechanical properties at 250° C.                                       Elastic modulus (MPa) 34 39  33 37                                            Yield point (MPa) 3.9 -- -- 3.9 --                                            Stress at break (MPa) 6.94 2.3  6.3 2.7                                       Elongation at break (%) 364 82  371 19                                        Mechanical properties at 275° C.                                       Elastic modulus (MPa)  25                                                     Yield point (MPa) -- -- -- -- --                                              Stress at break (MPa)  1.4                                                    Elongation at break (%)  9                                                    Creep at 275° C. and 1.3 MPa                                         Elongation at 15 min (%)                                                                         Immediate                                                                           A compressed                                           Elongation at 60 min (%) -- break under moulded -- --                         Elongation at 48 hours (%)  stress plaque was                                    not possible                                                                  to obtain                                                                __________________________________________________________________________     .sup.(*) comparative                                                     

What is claimed is:
 1. Tetrafluoroethylene (TFE) thermoprocessablecopolymers, consisting of:(A) from 0.1 to 15% by moles of a fluorodioxolof formula ##STR14## wherein R_(F) is a perfluoroalkyl having from 1 to5 carbon atoms; X₁ and X₂, equal to or different from each other, are--F or --CF₃ ; Z is selected from --F, --H and --Cl; (B) from 0 to 15%by moles of a perfluorinated monomer selected from hexafluoropropene(HFP) and a perfluoralkylvinylether of formula CF₂ ═CF--OR'_(f) whereinR'_(f) is a perfluoroalkyl C₂ -C₄ or their mixtures; and (C) TFE formingthe remaining part to 100%; with the proviso that the total amount ofthe monomers (A) and (B) is lower than or equal to 20% by mole, andexcluding the value 0 for the amount of the monomer (B).
 2. Copolymersaccording to claim 1, wherein the pefluoroalkylvinylether isperfluoropropylvinylether.
 3. Copolymers according to claim 1, whereinthe fluorodioxol of formula (I) is present in an amount comprisedbetween 0.5 and 9% by moles.
 4. Copolymers according to claim 1, whereinthe perfluorinated monomer (b) is present in amounts comprised between 0and 10% by moles.
 5. Copolymers according to claim 1, wherein the totalamount of the monomers (a) and (b) is lower than or equal to 12% bymoles.
 6. Copolymers according to claim 1, wherein in the formula (I) ofthe monomer (a) X₁ and X₂ are both --F.
 7. Copolymers according to claim1, wherein in the formula (I) of monomer (a) R_(f) is selected from--CF₃, --CF₅, and --C₃ F₇.
 8. Copolymers according to claim 1, having amelting viscosity such as to obtain Melt Flow Index (MFI) values,measured according to standard ASTM D 1238-52T, comprised between 0.5and 50 g/10'.
 9. Copolymers according to claim 8, wherein the Melt FlowIndex (MFI) value is comprised between 1 and 30 g/10'.